This study proposes to set up an artificial capillary culture system for thymic reticuloepithelial (TRE) cells, bone marrow and spleen reticuloendothelial (BMRE and SRE) cells, fetal liver (hepatic) parenchymoendothelial (FHPE) cells and control fibroblasts. These cells will be grown in the extra-capillary space of artificial capillaries perfused with tissue culture medium. The medium obtained from TRE cell or fibroblast cultures will be employed to condition cultures of T-cell deficient mouse precursor cells (obtained from thymectomized irradiated bone marrow protected radiation chimeras) and these cells tested pre- and post-conditioning for T cell characteristics employing a number of tests such as acquired O antigen and mitogen responsiveness. The medium obtained from TRE, BMRE, SRE and FHPE cell cultures, again using fibroblasts as a control, will be employed to condition hematopoietic stem cell concentrates prepared using nitrogen mustard pretreatment of donor mice followed by density gradient centrifugation and/or filtration. The stem cell concentrates will be tested pre- and post-conditioning for CFU-S, CFU-C and T and B cell content. Also the proliferative fraction, erythroid fraction and differential count will be determined. The perfusate will be assayed for granulocyte-monocyte colony activation factor (CAF). Hematopoietic stem cell concentrates will be circulated through these artificial capillary beds and characterized similarly. The objectives are to determine if the perfusate of the above stromal-cell cultures has lymphocyte and/or hematopoietic stem cell proliferation and differentiation-inducing properties, and, further, if hematopoietic stem cell concentrates circulated through such artificial capillaries show evidence of stimulation of proliferation and/or differentiation.